The present invention relates generally to lithium batteries, and more particularly to improvements in lithium batteries which allow them to be disposed of without injury to persons or property or harm to the environment.
Ambient temperature lithium batteries represent one of the very active, relatively new fields in battery technology. Lithium has low atomic mass, has the highest amp-hour capacity of any elemental metal in the periodic table, and this, coupled with its highly reactive nature, makes it a highly interesting and desirable anodic material in batteries. Primary non-rechargeable batteries based upon lithium metal anodes have been developed during the past 30 years. A large number of systems are presently in commercial production.
During the past two decades, researchers and battery manufacturers have promoted rechargeable lithium batteries using metallic lithium as a possible solution to many consumer electronics issues in the quest for higher power devices that can be lightweight and long lasting. A lithium anode battery offers many advantages, including high energy density. For example, in a AA size cell configuration, a lithium-manganese dioxide (Li/MnO2) primary or rechargeable battery has a capacity of 2 Watt-hours (Wh), compared to a nickel-cadmium (Nicad) battery which has a capacity of only 0.96 Wh in the same size cell configuration. Moreover, the lithium anode battery produces high voltage, twice that of a Nicad battery of the same configuration, and low self-discharge.
Unfortunately, however, the disadvantages of the lithium anode battery, thus far, have outweighed its advantages. These disadvantages include a need for expensive separators; a lower cycle life (in the rechargeable form) than the Nicad battery; a ruinous effect to the cell of discharging to zero volts; a relatively high cost; a long recharge time (typically, 10 hours); a safety factor that depends on cycling conditions; and a requirement of stack pressure on the lithium to undergoing cycling well. All of these problems are directly traceable to the lithium metal anode.
It is known that rechargeable lithium batteries using lithium metal anodes in contact with liquid organic electrolytes have many problems, most notably poor safety which limits battery size to smaller cells. Lithium is a very reactive element with most organic and inorganic electrolytes. The tendency for lithium to react with various solvents leads to film formation at the lithium/electrolyte interface and subsequent passivation problems.
The relatively poor cycling efficiency of the lithium anode arises because it is not thermodynamically stable in typical non-aqueous electrolytes. Cells contain typically three-to five-fold excess lithium to ensure a reasonable cycle life (number of times the battery can be recharged). Lithium plating and stripping during the charge and discharge cycles creates a porous deposit of high surface area and increased activity of the lithium metal with respect to the electrolyte. The reaction is highly exothermic and the cell can vent with flame if heated.
But these reactions can be alleviated or reduced by using alternative anodes. Many lithium alloys with lower reactivities have been proposed in the literature. Lower lithium reactivity should lead to increased safety, avoiding problems with rechargeable lithium cells attributable to the high reactivity of the lithium anode with the accompanying solvent. In any event, rechargeable lithium batteries using metallic lithium anodes in organic solvents are no longer manufactured. Instead, carbon anodes that intercalate lithium ions are the present focus of commercial rechargeable lithium ion batteries but contain no free lithium metal in the anode structure.
In addition to the aforementioned issue of safety of primary and rechargeable lithium cells, other safety issues abound such as disposability and recyclability of lithium batteries. Disposing of cells containing free lithium metal into an incinerator can lead to explosions, just as crushing these cells may also lead to fire and explosions. Furthermore, recycling of such cells requires careful handling; otherwise, the unspent lithium could lead to these same results.
The Gibb""s Free Energy change for a cell reaction,
Li+X=LiX
may be calculated from thermodynamic data for many cathode couples where X represents the cathode. The cathode may be one that intercalates lithium ions, for example, vanadium oxide (V6O13), manganese oxide (MnO2) cobalt oxide (CoO2) and forms a single lithiated cathode compound with lithium ions, for example Li8V6O13 or LiMnO2 or LiCoO2, or it can be one such as carbon monofluoride, CFx, that can form a free salt with the lithium ions to form lithium chloride, LiCl and free carbon.
The battery capacity of a particular cell is usually determined by the capacity of the cathode. For example, a V6O13 cathode that can intercalate 8 lithium ions into its structure has a capacity of 417 milliamp-hours per gram (mAh/g) while a CFx cathode forming one mole of LiCl has a capacity of 1.033 Ah/g; typically, lithium would have a capacity of 3.86 Ah/g. Because of the high electrochemical reactivity of metallic lithium in non-aqueous electrolytes, all types of primary lithium batteries have been manufactured with limited cathode capacity and excess anode capacity, so that the maximum cell capacity can be extracted during cell discharge. The excess lithium also accounts for any parasitic reactions that could occur between the lithium metal and the liquid solvent electrolyte when the cell is on standby. Clearly, the emphasis has been on cell performance, and not particularly on safety or disposal issues.
Since approximately 1970, primary lithium batteries have been manufactured in small cells principally for photographic uses, and in more limited quantities in watches and circuit boards. Larger cells such as those based on thionyl chloride and sulfur dioxide cathodes have been used mainly in military applications such as for communications devices, and in oil drilling operations, among other applications. All of these applications have addressed problems of disposal and safety, but heretofore, practical solutions to these problems have not surfaced. Military battery usage, in particular, has encountered great difficulty in disposal protocols.
Primary lithium batteries are now being used or considered for use in a large number of consumer products, such as smart cards, greeting cards, ornaments, and other consumer applications, as well as finding increased use in applications in the military, medical field, and oil drilling, among others. In consumer applications such as greeting cards and smart cards, the assumption is that the consumer will have depleted the battery power before disposing of the battery, and in certain other consumer applications, before disposing of the device in which the battery is used. Nevertheless, free lithium usually remains in the spent battery, so that disposal of the battery can pose safety issues.
According to the present invention, a primary lithium metal anode battery is designed to have either a balanced anode and cathode capacity, or to be anode capacity limited. As a result ofthis design, when all of the anode material has been consumed (i.e., fully reacted with the cathode), the reaction that produced this consumption forms either a discharged lithium ion intercalated cathode product or a harmless lithium salt as a by-product, a finite cathode capacity still remains unconsumed. Typically, the discharged cathode by-product in whatever form is usually benign, non-toxic, environmentally disposable, and safe from explosions, fire and other potential hazards.
Although this design may result in a shallower discharge, no significant quantity of free lithium remains in the cell. This means that the cell can be incinerated or otherwise disposed of safely, since the final products of the cell are then solvent, separator, salt, substrate material and benign cathode product. By carefully balancing the anode and cathode capacity of the cell or making the cell anode capacity limited in accordance with the teachings of the present invention, cell capacity can be maximized or nearly so, and yet without residual free lithium upon complete discharge of the cell. In addition, designing primary lithium batteries with reduced lithium content will lead to a reduction in the materials cost of the battery as lithium is a very expensive metal.
According to another aspect of the invention, the primary lithium battery is designed to incorporate a lithium alloy such as LiSlx, LiAlx, LiAlxCry, LiAixMny, or the like, also with either balanced anode and cathode capacity, or anode limited. The outcome of a complete discharge would be quite similar to that described above, except that the parasitic reaction between the anode and electrolyte would also be minimized during standby.
Therefore, it is a principal object of the present invention to provide a lithium battery having an internal design to assure that the battery, when spent, will have virtually no free lithium to adversely affect its safe disposal.
Another object of the invention is to employ a lithium alloy in the battery to provide a safer lithium anode regardless of whether some lithium alloy remains after complete discharge.
In the case of large primary batteries produced according to the invention, in usage in military or industrial applications the battery may be allowed to discharge completely so that all the free lithium is consumed, just as with smaller lithium cells used for consumer applications, whereby to enable either disposal or recycling of the battery in a safe manner.
In presently preferred embodiments of the invention, the lithium is laminated either on a planar metal substrate or gauze of a suitable metal such as nickel, aluminum, copper or carbon. This assures that upon complete consumption of the lithium, the integrity of the anode substrate remains intact. The tab connection to the substrate of the anode needs to be on a rigid support; otherwise a tab connecting directly to the lithium metal or its alloys may lose contact and hence integrity during discharge, resulting in loss of electrical contact and partial cell discharge and free lithium remaining in the cell.
Additionally, the capacity of the lithium is controlled in preferred embodiments by vacuum depositing the lithium anode into a thin film to reach the desired capacity.